denuseri is correct - the dom has to be ultra attentive. I've seen them watching their partner so closely the audience is ignored.

I like the stoplight technique. The Dom watches the sub and every few minutes when the sub seems lost/out of breath/over-whelmed you ask "What color is the light?".

Red light - stop/end/get me out of here.
Yellow - What is happening now is too much or un-intentional pain. Reduce it or try something else
Green - Gimme more.